Permanent or Electro Overband Magnet Selection

By Paul Fears | 11 January 2021

The Overband Magnet is one of the most commonly used magnetic separators.  Different designs are used in nearly all process industries and especially in mining, quarrying, recycling, and bulk handling applications.

Inline overband magnet at work
Permanent Overband Magnet in a household waste recycling plant

In operation, an Overband Magnet (or Suspension Magnet) sits above a conveyor carrying material.  As the material passes under the magnet, ferrous metal is attracted up and out of the non-magnetic burden.  Such metal separation is either to recover ferrous metals (e.g. recovery of steel cans) or to remove tramp iron for the protection of processing plant from damage (e.g. such as crushers, shredders and screens).

The two designs of Overband Magnet are categorised by the method of generating the magnetic field.  These are:

  1. Permanent Overband Magnets using a permanent magnetic block, commonly constructed from ceramic ferrite magnets;
  2. Electro Overband Magnets using either an air or oil cooled electromagnetic coil;

Overband Magnet Selection Criteria

Both the Permanent and Electro Overband Magnets have advantages and disadvantages.  Selection is always application specific, but related to the installation as well as the separation objective.  Bunting’s applications engineers consider a number of key factors when assessing the best Overband Magnet for a specific application including:

Ferrous Metal Separation Objective

Generally, the separation objective will either be ferrous metal recovery or tramp iron removal.  Removing tramp iron to protect other processing plant such as crushers, shredders, and screens is critical and requires the most powerful magnetic separator.  Hence, Electro Overband Magnets are commonly selected for such applications.  Although important, ferrous metal recovery is not as critical leading to the recommendation of Permanent Overband Magnets.

Ferrous Metal Separation Tests on an ElectroMax Overband Magnet

Ferrous Metal Size and Shape

Long ferrous metal rods and whole steel used beverage cans are easier to separate than small almost spherical ferrous parts like nuts.  The shape and nature of the Permanent Overband Magnet magnetic field makes the design ideal for separating those larger ferrous metals.

Electro Overband Magnets are preferable for the critical separation of tramp iron in the form of nuts, bolts and ferrous metal blasting caps.  This protects processing plant including crushers, screens and conveyors, as well as ensuring the end product is metal-free (which is critical for many mining applications such as in the coal industry).  The magnetic attractive force of an Electro Overband Magnet is stronger at distance than a permanent design, with a higher magnetic gradient attracting and extracting smaller ferrous metals.

Deep Burden Depth

When there is a deep burden of conveyed material, high magnetic forces are needed to extract the ferrous metal from underneath the non-metallic material.  Also, the deep burden depth means a high suspension height (i.e. the distance between the base of the Overband Magnet and the bottom of the burden).  The optimum deep and strong magnetic field is produced by an Electro Overband Magnet.  Permanent Overband Magnet designs, able to produce deep and strong magnetic fields, are exceptionally large and heavy, making installation costly and difficult.

Overband Magnet Weight

For many installations, such as on mobile plant, weight is a critical factor.  Permanent Overband Magnets are lighter than Electro Overband Magnets, especially for narrower conveyors where the suspension height is between 200 and 300mm.

However, as the required suspension height rises, so does the size and weight of the permanent magnet block, to a point where it exceeds that of an equivalent Electro Overband Magnet (such as the ElectroMax). This is because the additional magnetic force at distance can only be generated by adding blocks of permanent magnets (e.g. ceramic ferrite) and potentially increasing the size and weight of the solid-steel back-bar.

The 2020-launched lighter and more compact range of ElectroMax Overband Magnets bridges the gap between Permanent and Electro Overband Magnets.  The stronger magnetic field of the ElectroMax enables enhanced tramp iron separation.

Bunting ElectroMax Overband Magnet
The ElectroMax-Plus Overband Magnet

Electrical Power

Permanent Overband Magnets only need power to drive the self-cleaning belt motor, using either an electric or hydraulic motor.  The electromagnetic coil of an Electro Overband Magnets requires power to generate the magnetic field.  In some installations, and especially on mobile plant, there are difficulties in accessing such power leading to the preferred use of permanent designs.

Magnet On or Off

As the product name suggests, a Permanent Overband Magnet permanently generates a magnetic field.  Therefore, for applications where the Overband Magnet requires switching off, the only option is the Electro Overband Magnet, where the magnetic field is easily switched on and off.

Application Review

As with many applications, understanding the installation and separation objective is key to a successful selection. Bunting’s applications engineers have many years of experience in Overband Magnet selection and use bespoke computer modelling to determine the optimum size for any given project.

For further information on Bunting’s range of Permanent and Electro Overband Magnets or to discuss a specific project, please contact our technical sales team on:

Email: Gordon Kerr at GKerr@buntingmagnetics.com

Via the website

Telephone:  +44 (0) 1527 65858

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